Oculopharyngeal muscular dystrophy (OPMD) is a late onset disorder causing weakness of eyelid and pharyngeal, and proximal limb muscles. A major symptom of OPMD is swallowing difficulty (dysphagia) due to weakened pharyngeal muscles. Patients often experience choking or aspiration pneumonia, and may require a feeding tube in later stages of disease. At present, the exact molecular mechanisms of pathology are unknown. The only available options to treat dysphagia in OPMD patients are temporary, usually surgical procedures to decrease resistance in the throat to improve swallowing. Autosomal dominant OPMD patients harbor a mutation in the gene for the nuclear mRNA processing protein PABPN1, which causes the N-terminal polyalanine stretch of PABPN1 to expand from 10 to 11-18 residues. PABPN1 plays key roles in mRNA processing including polyadenylation of nuclear transcripts and the alternative polyadenylation and cleavage pathway. PABPN1 is expressed in all tissues, but mutation causes late-onset pathology only in a specific subset of skeletal muscles. How mutation of this ubiquitously expressed protein leads to late-onset pathology in specific tissues remains an open question. Skeletal muscle tissue is capable of regeneration. Closely associated with multinucleated, post-mitotic myofibers in adult muscles is a pool of stem cells known as satellite cells. Upon muscle damage, satellite cells become activated to proliferate, differentiate, and fuse with each other and existing myofibers. This process, known as myogenesis, restores muscle architecture and function. Interestingly, some of the muscles most severely affected by OPMD, including pharyngeal muscles, are more highly regenerative than other skeletal muscles. The hypothesis driving the work proposed here is that mutant PABPN1 disrupts mRNA biogenesis and decreases the myogenic capacity of pharyngeal satellite cells, leading to pathology due to the unusually high demand for myogenesis in pharyngeal muscles. We will take an integrative in vitro and in vivo approach using a novel mouse model that is the first to accurately copy the genotype of OPMD patients. This mouse model expresses mutant PABPN1 in all tissues. Because PABPN1 is known to impact mRNA metabolism, satellite cells from pharyngeal muscles will be isolated and assayed for RNA phenotypes (Aim 1). The myogenic program can be recapitulated in vitro and used to quantify specific stages of myogenesis. Therefore, primary activated satellite cells, known as myoblasts, from pharyngeal muscles will be isolated and used to quantify the effects of mutant PABPN1 on the specific stages of myogenesis in vitro (Aim 2). This model will then be used to assay the effect of mutant PABPN1 on myogenesis in pharyngeal muscle in vivo (Aim 3). The long-term objective of this work is to understand how mutant PABPN1 affects pharyngeal satellite cell biology and potentially identify targets for new pharmacologic OPMD treatments.